Mainstream society 倒也偶尔能看见，google了一下，觉得就是指the general public：

No longer confined to technology fans and those ahead of the curve, MP3 players are entering mainstream society as prices fall and general consumers get used to them.
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While no specific lifestyle or practice is associated with the mainstream, the concept does imply participation in socially acceptable behaviour and the avoidance of activities that are deemed unacceptable or illegal.
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The headline was "Polygamists Fight to Be Seen As Part of Mainstream Society."